Typically it is dirty passages inside the carburetor. Basically fuel flow is slightly restricted so the carb is working outside of its normal calibration - the governor is not controlling well outside the normal calibration. This is most common after long winter of no use.
I use carburetor cleaner. I carefully disassemble things like the fuel bowl (preserving the gaskets) and spray cleaner down the jets, through every air passage etc. (there are small air bleed holes in the carb venturi). Wear eye protection because it will squirt back at you sometimes!
Mixture screws... Screw them all the way in, counting how many turns until they bottom out. Then screw them all the way out and remove them. Spray cleaner down there as well. Re-assemble screws by bottoming them out and backing out the number of turns you counted.
Never use drills or wires to clean out passages on a carburetor. Hard metals will wear on soft brass fittings. Carburetors are calibrated! Carb cleaner in a spray can is relatively safe.
It will run 100% better (and start better as well).
Next time use fuel stabilizer (I put it in every tank because I can never predict when the 'last' time I will use it will be).
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